In the current state of operations, occasions arise such that threats (or targets) must be detected at a standoff distance. Some threats contain components whose permittivity contrasts substantially with that of the emplacement; such is the case with many threats that are buried. The reception of a subsurface linear radar response from an area whose surface is otherwise undisturbed indicates the presence of a threat. Others threats contain metal contacts and semiconductor junctions whose nonlinear electromagnetic response contrasts with that of the emplacement; such is the case with radio frequency (RF) electronics. The reception of a nonlinear radar response from an area that does not otherwise contain electronics indicates the presence of another class of threat. Often, threats contain dielectric, as well as electronic components, hence they will respond to both linear and nonlinear excitation and either linear or nonlinear radar with detect such as threat. Hence, there exists a need to detect both types of threats, whether or not they are collocated using a single assembly or unit.
In the publication by D. Young, et al., entitled “Ultra-Wideband (UWB) Transmitter Location Using Time Difference of Arrival (TDOA) Techniques,” Conference Record of the 37th Asilomar Conference on Signals, Systems and Computers, (2004), 1225-1229 Vol. 2, DOI: 10.1109/ACSSC.2003.129218, techniques to determine the location of a transmitter are disclosed. In the publication by H. Leung and X. Liu, entitled “Resolution enhancement in passive SAR imaging using distributed multiband fusion,” Proceedings of 2013 IEEE Antennas and Propagation Society International Symposium (APSURSI), 2013, 1026-1027, DOI: 10.1109/APS.2013.6711173 (hereinafter Leung), the usage of synthetic aperture radar imaging using multiband fusion is disclosed. According to the Leung abstract, using multiple emitters, a ‘fusion-before-imaging’ method for passive synthetic aperture radar (SAR) imaging is utilized to effectively fuse target signature data from multiple receivers in order to achieve a synthesized target reflectivity with a broader bandwidth coverage. The Leung method jointly compensates the phase offsets among sub-band signals due to geographically varied bistatic radar configurations and estimates the missing spectral data to mitigate the image artifacts in conventional direct Fourier based imaging technique.